In view of global environmental protection, boiler-type heating appliances for heating by burning fossil fuel have been replaced by heat-pump-type air-conditioning apparatuses using air as heat sources in more and more cases even in cold regions in recent years. The heat-pump-type air-conditioning apparatus can efficiency perform heating because heat is supplied from air in addition to an electrical input to a compressor.
On other hand, in the heat-pump-type air-conditioning apparatus, however, frost is more easily accumulated on an outdoor heat exchanger serving as an evaporator as the temperature of air in, for example, the outside (outdoor-air temperature) decreases. Thus, it is necessary to perform defrosting (frost removal) for melting frost on the outdoor heat exchanger. For such defrosting, an example method is to reverse a refrigerant flow in heating so as to supply refrigerant from a compressor to an outdoor heat exchanger. This method, however, is performed while heating of a room is stopped in some cases, and thus, has the problem of a loss of comfort.
In view of this, to perform heating during defrosting, proposed are methods for heating by dividing outdoor heat exchangers in such a manner that while some of the outdoor heat exchangers are defrosted, the other outdoor heat exchangers operate as evaporators so as to absorb heat from air, for example (e.g., Patent Literature 1, Patent Literature 2, and Patent Literature 3).
For example, in a technique described in Patent Literature 1, an outdoor heat exchanger is divided into two heat exchanger parts. Then, to defrost one of the heat exchanger parts, an electronic expansion valve disposed upstream of this heat exchanger part is closed. In addition, an electromagnetic shut-off valve of a bypass pipe for conveying refrigerant from a discharge pipe of a compressor to an inlet of the heat exchanger part for bypassing is opened so that part of high-temperature refrigerant discharged from the compressor flows directly into the heat exchanger part to be defrosted. When defrosting of one of the heat exchanger parts is completed, defrosting of the other heat exchanger part is performed. In this case, in a heat exchanger part to be defrosted, defrosting is performed in a state in which the pressure of refrigerant in this heat exchanger part is substantially equal to a suction pressure of the compressor (low-pressure defrosting).
In a technique described in Patent Literature 2, a plurality of heat source units and at least one indoor unit are provided, and refrigerant discharged from a compressor is caused to flow directly into a heat source unit side heat exchanger to be defrosted by reversing connection of a four-way valve of only a heat source unit including the heat source side heat exchanger to be defrosted. In this case, in the heat source unit side heat exchanger to be defrosted, defrosting is performed in a state in which the pressure of refrigerant in this heat source unit side heat exchanger is substantially equal to a discharge pressure of the compressor (high-pressure defrosting).
In a technique described in Patent Literature 3, an outdoor heat exchanger is divided into a plurality of outdoor heat exchanger parts in such a manner that part of high-temperature refrigerant discharged from a compressor alternately flows into the outdoor heat exchanger parts so as to alternately defrost the outdoor heat exchanger parts. Thus, heating can be continuously performed without reversing a refrigeration cycle. Refrigerant supplied to an outdoor heat exchanger part to be defrosted is injected from an injection port of the compressor. In this case, in the outdoor heat exchanger part to be defrosted, defrosting is performed in a state in which the pressure of refrigerant in this outdoor heat exchanger part is lower than a discharge pressure of the compressor and higher than a suction pressure of the compressor (a pressure that is slightly higher than 0 degrees C. in terms of saturation temperature) (medium-pressure defrosting).